Flaviviruses and alphaviruses are spread by mosquito and tick vectors and cause severe human and animal illnesses such as encephalitis and hemorrhagic fever. These viruses include many potential bioterrorist agents that are category A-C pathogens, such as the flaviviruses dengue, West Nile, Japanese encephalitis and yellow fever viruses, and the alphaviruses Venezuelan, eastern, and western equine encephalitis viruses. Dengue virus (DV) is currently of particular concern as it has dramatically reemerged to become endemic in more than 100 countries including the US, and is now a global health problem. There are an estimated 50-100 million cases of dengue fever and 500,000 cases of the more lethal complication dengue hemorrhagic fever per year, with significant impact on both human health and the economies of developing countries. Antiviral strategies for the flaviviruses and alphaviruses are urgently needed. The flavivirus and alphavirus membrane fusion proteins are members of the class II virus fusion proteins. They are structurally very similar and refold to a homotrimer form to mediate virus fusion and infection. In collaboration with Dr. F[unreadable]lix Rey, we have recently determined the structure of the homotrimer conformation of the fusion protein from the alphavirus Semliki Forest virus (SFV). The SFV homotrimer structure is strikingly similar to that of DV. Using the structure as a guide, we have developed protein fragments that act as specific dominant-negative inhibitors of SFV and DV fusion and infection. Based on this information, we now plan to develop a general screen for inhibitors of class II fusion reactions. In aim 1 we will establish and optimize in vitro methods to follow the protein-protein interactions during the class II fusion protein refolding reaction. In aim 2, we will adapt this assay to a high throughput format, and use it to screen peptide and small molecule libraries available at our institution and through the NIH and Northeast Biodefense Consortium. Ultimately, such inhibitors will be lead compounds for antiviral therapy, and important research tools to understand the class II virus fusion reaction. Flaviviruses and alphaviruses include many important human pathogens and potential bioterrorist threats such as the flaviviruses dengue, West Nile, Japanese encephalitis and yellow fever viruses, and the alphaviruses Venezuelan, eastern, and western equine encephalitis viruses. Dengue virus is currently of particular concern as it has dramatically reemerged to become endemic in more than 100 countries including the US, and is now a global health problem. This application focuses on developing new antiviral strategies for the flaviviruses and alphaviruses, based on blocking the activity of the proteins involved in the initial entry of the virus into the cell. [unreadable] [unreadable] [unreadable]